1. Field of the Invention
The present invention relates to an industrial process for the preparation of alkyl nitrites from nitrogen oxides, oxygen and lower alkanols, the nitrogen oxides having a proportion of more than 50% of NO of the total amount of gram atoms of N of the nitrogen oxides being used. Some of the lower alkanol is used in the lower part of the reactor together with the nitrogen oxides and the remaining part is charged in the upper part of the reactor.
Alkyl nitrites (alkyl esters of nitrous acid) have a varied use, for example as additives for motor oils, as stabilizers for unsaturated organic compounds, as spasmolytics, as reagents for oximations, nitrosations and diazotizations and as intermediates for chemical syntheses.
2. Description of the Related Art
It has long been known that alkyl nitrites, including in particular methyl nitrite, can be prepared by reaction of nitrogen oxides with the corresponding alkanols, including in particular methanol, with the participation of an amount of oxygen adapted to the amount and composition of the nitrogen oxides used. For example, in the preparation of methyl nitrite from nitrogen oxides, oxygen and methanol, fundamentally the following reactions shown can proceed between the participating components: EQU 2 NO+O.sub.2 .fwdarw.2 NO.sub.2 ( 1) EQU NO.sub.2 +NO.revreaction.N.sub.2 O.sub.3 ( 2) EQU R--OH+N.sub.2 O.sub.3 .fwdarw.R--ONO+HNO.sub.2 (R.dbd.CH.sub.3) (3) EQU R--OH+HNO.sub.2 .fwdarw.R--ONO+H.sub.2 O (4) EQU N.sub.2 O.sub.3 +H.sub.2 O.fwdarw.2 HNO.sub.2 ( 5) EQU 2 NO.sub.2 .revreaction.N.sub.2 O.sub.4 ( 6) EQU R--OH+N.sub.2 O.sub.4 .fwdarw.R--ONO+HNO.sub.3 ( 7) EQU N.sub.2 O.sub.4 +H.sub.2 O.fwdarw.HNO.sub.2 +HNO.sub.3 ( 8)
In the preparation of the target alkyl nitrites, the procedure is preferably followed so that as far as possible only the reactions reproduced in reaction equations (1) to (4) proceed and water is obtained as the sole waste material. The reaction reproduced in reaction equation (5) is generally unavoidable since the added dinitrogen trioxide, and that formed according to reaction equation (2) can react to completion not only with the alcohol in accordance with reaction equation (3), but also with the water resulting according to reaction equation (4). However, in the presence of sufficient, in particular excess, amounts of alcohol, the nitrous acid resulting in this case in accordance with reaction equation (4), is scavenged with formation of the desired alkyl nitrite and water, that is is not lost for example as waste material.
The reactions reproduced in the reaction equations (6) to (8) are undesirable since they lead to the irreversible formation of nitric acid. Formation of this by-product reduces the yield of desired alkyl nitrite, based on nitrogen oxide used. The resulting nitric acid must be separated off and, in particular, on carrying out the overall process on an industrial scale, waste waters are produced in this case which must receive secondary treatment with not insignificant expenditure. In order to exclude as far as possible the dimerization proceeding according to reaction equation (6) of the nitrogen dioxide either added in a mixture with nitrogen monoxide or formed via equation (1), which dimerization initiates the formation of nitric acid, the stoichiometric ratio of nitrogen monoxide to nitrogen dioxide is preferably adjusted so that it adopts a value &gt;1. In this manner the formation of dinitrogen tetroxide is successfully repressed in favour of dinitrogen trioxide.
As can be seen from the reaction equations (1) to (3), during the feed-in of nitrogen oxides to prepare alkyl nitrites, the following procedure can be advantageously followed for example:
Nitrogen monoxide and nitrogen dioxide are fed in separately or as a mixture, a molar ratio NO:NO.sub.2 of &gt;1 occurring, which according to the above statements likewise reduces the probability of formation of dinitrogen tetroxide and, in association with this, has as a consequence the repression of the formation of nitric acid. PA1 Oxygen is additionally fed in, separately from the nitrogen oxides or as a mixture with them, more precisely preferably in a molar ratio NO:O.sub.2 of &gt;4, based on the number of moles of NO, which exceeds the number of moles of NO.sub.2 used, which in correspondence with the above statements has the consequence of preference of the formation of dinitrogen trioxide in comparison to that of dinitrogen tetroxide and, associated with this, a repression of the formation of nitric acid. PA1 (A) The preparation of dimethyl oxalate from carbon monoxide in the presence of suitable catalysts PA1 [cf. EP 46 598] ##STR1## (B) The oxidation of unsubstituted or substituted olefins in the presence of methanol and suitable catalysts PA1 [cf. EP 55 108] ##STR2## (C) The preparation of dimethyl carbonate from carbon monoxide in the presence of suitable catalysts [cf. EP 425 197 and Zeitschrift f. Katalyt. Forschung, (China), Vol. 10(1), p. 75-78 (1989)] ##STR3## PA1 The oxygen fed in for regenerating the methyl nitrite from the returned nitrogen monoxide is to be reacted as completely as possible to form methyl nitrite. PA1 This applies equally to all higher nitrogen oxides situated within the methyl nitrite generator, namely to nitrogen dioxide, dinitrogen trioxide and dinitrogen tetroxide. PA1 The reaction conditions and the reaction technique are to be optimized so that the undesirable formation of the by-product nitric acid (cf. reaction equations (7) and (8)) is excluded as far as possible. This simultaneously serves for a maximum possible yield of desired alkyl nitrite. PA1 The complete reaction of the fed oxygen or of the higher nitrogen oxides to give the desired alkyl nitrite is to take place within the alkyl nitrite generator. This avoids, in the case of an incomplete reaction of the participating components within the reaction space provided, complete reaction only occurring downstream of the alkyl nitrite generator and the reaction water (cf. reaction equation (4)) formed in this case being carried over into any downstream reaction which can possibly lead to undesirable side reactions there. PA1 The reaction water resulting in the course of formation of the desired alkyl nitrite and the nitric acid (cf. reaction equations (7) and (8)), possibly formed as a consequence of undesirable side reactions, are to be separated off as completely as possible from the gaseous product stream which leaves the alkyl nitrite generator. PA1 The heat of reaction released in the course of the reaction proceeding within the alkyl nitrite generator is to be conducted away. PA1 In view of the safety requirements of the industrial execution of processes in which the alkyl nitrite generators of the principal mentioned here are integrated, local overheating and the formation of ignitable mixtures must be avoided.
A series of industrially interesting continuous processes is known in which the formation of, for example, methyl nitrite in the gas phase proceeds within an overall process, which is characterized in that, in it, methyl nitrite functions as an oxidizing agent, cocatalyst, alkoxylation reagent or other type of reaction partner. It is typical for this process that in its course, the nitrogen monoxide formally contained in the methyl nitrite is not consumed, but is liberated as a gas. In a preferred embodiment of reactions of this type, the freshly generated methyl nitrite is introduced together with the additional reaction partner or reaction partners and reaction auxiliaries into the reaction space or the reaction vessel and the portion of the product gas stream remaining in the gaseous state which contains the nitrogen monoxide formed in the course of the reaction is returned to the reactor for the methyl nitrite preparation after separating off as far as possible the condensed or condensable reaction product or reaction products. This closes a cyclic process, based on nitrogen monoxide and methyl nitrite. The principle of such a process is depicted in FIG. 1.
Reactions in which such a continuous cyclic procedure is particularly advantageous are for example the following:
The reactions mentioned are carried out in the gas phase or alternatively in the liquid phase. In the latter case, methanol is preferably used as solvent. It is generally characteristic that both the methyl nitrite fed and also the nitrogen monoxide formed are supplied and conducted away, respectively in the gaseous state. The gas circulated can, depending on the selected reaction conditions, the concentration and the physicochemical characteristics of the participating components as well as the phase equilibrium contain still further substances beyond methyl nitrite and/or nitrogen monoxide. Inert gases, such as carbon dioxide, nitrogen and argon, reactants such as carbon monoxide and olefins (cf. (B), for example methyl acrylate), reaction products, such as dimethyl oxalate (cf. (A)) and dimethyl carbonate (cf. (C)) as well as methanol dissolved in the gas stream can be mentioned here by way of example.
Dimethyl oxalate is of great interest as an intermediate, for example for the preparation of ethylene glycol, which itself has broad use as solvent, coolant and as raw material for the fabrication of polyester fibres and alkyd resins.
Acetals, as described under (B), are important starting materials for the preparation of heterocycles, which are themselves used as intermediates in the synthesis of pharmaceutical and agrochemical active compounds.
Dimethyl carbonate is used as a less toxic methylation reagent, in comparison to, for example methyl chloride or dimethyl sulphate, as a propellant additive and as starting material for the preparation of diphenyl carbonate. Diphenyl carbonate serves as an intermediate for the preparation of isocyanates and polycarbonates.
Taking the requirements below into account is essential for the successful operation of continuous processes as mentioned in the examples (A), (B) and (C):
All the requirements mentioned are satisfied by the process according to the invention in a simple manner surpassing the prior art.
In Patent Application EP 310 191, which describes quite generally a process for the preparation of alkyl nitrites, in particular methyl nitrite and ethyl nitrite, with participation of oxygen in the reaction, it is proposed to configure the entire reaction vessel, in which the reaction between oxygen, nitrogen monoxide and the particular alcohol takes place, as a scrubber having two spatially separated zones, a reaction zone and a rectification zone. The scrubbing medium, preferably identical to the alcohol brought to the reaction and used as such in stoichiometric excess, is fed in by the counter-current principle at the head of the rectification zone and conducted away in opposite direction to the rising product gas stream composed inter alia of the alkyl nitrite and the water produced during its formation. A disadvantage in this case is the separate layout of reaction zone and rectification zone which requires a high expenditure in terms of apparatus.
According to the patent application mentioned, the rectification zone is laid out in the form of a tray column. A disadvantage of such a procedure is that a completely mixed gas zone is present above each tray, back mixing effects occurring. In order, despite this, to realize the desired separation effect, that is the removal of the water produced in the course of formation of the alkyl nitrite and the water-soluble by-products, such as for example nitric acid, the entire apparatus must be dimensioned significantly larger than would be possible with the exclusion of back-mixing effects. It can be noted that, for example, any nitric acid contained in the product gas stream leaving the alkyl nitrite generator, or even water, can interfere even in the smallest amounts highly sensitively with the downstream process, for example the preparation of dimethyl oxalate or dimethyl carbonate.
In the same patent application it is described that the heat of reaction released in the production of the alkyl nitrite can be removed by withdrawing from the reaction zone a liquid side stream which, after external cooling, is fed back into a higher-situated part of the reaction zone. However, apart from an increased expenditure in terms of apparatus, this has the additional disadvantage that the concentration and the residence time of the reaction water formed within the reaction zone are thereby increased and prolonged, respectively. In this case, on the one hand an increased formation of by-products can occur, on the other hand, the use of increased amounts of scrubbing liquid, especially of methanol in the case of the methyl nitrite preparation, and possibly a further enlargement of the rectification zone are required in order to keep the product gas mixture exiting at the head of the overall reactor as far as possible water-free.
Finally, all data which are to be inferred from the Patent Application EP 310 191 mentioned are based only on computer simulation calculations. The only example, likewise based on such a computer simulation is not reproducible for those skilled in the art, since firstly, the mass flow rates entering and exiting, based on the alkyl nitrite reactor, are not specified and, secondly, no data are given on the reaction volumes, which alone decide on the residence time required by the reaction kinetics given. The actual efficiency of the arrangement of apparatus described is therefore insufficiently documented and can thus not be evaluated at all. Furthermore, the example mentioned is also irrelevant from industrial aspects, since it apparently contains no rendering inert of the fundamentally ignitable alkyl nitrite stream and therefore the carry-over effect due to the inert gas portion occurring when a procedure of sufficient safety is used is not taken into account. Since, for example, the lower explosion limit of the system alkyl nitrite/nitrogen monoxide/carbon monoxide/alcohol/inert gas is shifted towards lower alkyl nitrite concentrations with increasing pressure, this question is of highly decisive importance in cyclic processes, such as for example the industrial preparation of dimethyl oxalate or dimethyl carbonate, in which such gas mixtures are fed into alkyl nitrite reactors.
The object was therefore still to find a process for the continuous preparation of C.sub.1 -C.sub.4 -alkyl nitrites from the underlying C.sub.1 -C.sub.4 -alkanols, oxygen and nitrogen oxides, which takes into account the abovementioned requirements and is suitable for integration into continuous processes in which C.sub.1 -C.sub.4 -alkyl nitrites are consumed with release of NO.